A method for preparing electrowinning nickel using crude nickel sulfate, a byproduct of copper electrolysis.

Abstract

This invention provides a method for preparing electrowinning nickel using crude nickel sulfate, a byproduct of copper electrolysis. The method includes the following steps: dissolving crude nickel sulfate in tap water, controlling the nickel concentration at 80-110 g / L, and filtering out insoluble calcium sulfate; adding an oxidant at 70-90°C, stirring for 2 hours, adjusting the pH to 5-5.5, and filtering to obtain a preliminary purified solution; taking the preliminary purified solution and electrowinning it using a low current to remove copper impurities, obtaining a copper-removed solution; taking the copper-removed solution and electrowinning it to remove zinc impurities, obtaining a purified solution; the impurities in the crude nickel sulfate are, in descending order, calcium, zinc, copper, iron, cobalt, lead, and arsenic; the pH of the prepared crude nickel sulfate solution is <2, and the pH is adjusted entirely using nickel carbonate; electrowinning the purified solution to prepare electrowinning nickel. The method provided by this invention is simple and avoids the environmental and human hazards caused by using sodium sulfide or hydrogen sulfide under acidic conditions. By purifying and removing impurities from crude nickel sulfate containing multiple metals, it meets the requirements for electrowinning nickel, thereby obtaining high-grade electrowinning nickel.

Description

Technical Field

[0001] This invention belongs to the field of nickel sulfate refining technology, and particularly relates to a method for preparing electrolytic nickel using crude nickel sulfate, a byproduct of copper electrolysis. Background Technology

[0002] During copper electrolytic refining, the nickel content in the electrolyte gradually increases with continuous circulation. When the nickel concentration reaches a certain level, it must be treated by open-circuit processing, using a concentration and crystallization method to obtain crude nickel sulfate as a byproduct. The raw material crude nickel sulfate composition is: Ni 18–21%, Ca 0.5–1%, Cu 0.2–0.5%, Zn 0.2–0.5%, Fe 0.05–0.3%, Co 0.05–0.1%, Pb 0.01–0.05%, and As 0.01–0.05%. Because crude nickel sulfate contains impurities such as calcium, copper, zinc, iron, cobalt, lead, and arsenic, it does not meet the national standards for Class I and Class II nickel sulfate. Furthermore, the presence of arsenic and lead results in a low nickel pricing coefficient, cheap external sales price, and high inventory pressure, leading to certain economic losses for production enterprises.

[0003] The traditional processing route for crude nickel sulfate involves purification and impurity removal to produce refined nickel sulfate. Reported methods for refining nickel sulfate include: chemical precipitation, involving sulfide precipitation, oxidation, quicklime hydrolysis precipitation, fluoride precipitation, concentration, and crystallization to obtain the product. While these methods achieve acceptable impurity removal rates, they result in significant nickel loss, large quantities of hazardous waste containing lead and arsenic, the generation of toxic gases, high reagent consumption, and a high sodium content in the product; solvent extraction, using P204 or P507 extractants to remove impurities, followed by concentration and crystallization. This method relies heavily on the precise control of the organic solvent, diluent, and water phase ratios, depending on operator experience. It generates large amounts of wastewater and organic waste liquid, the volatile organic reagents create a poor working environment, and calcium and magnesium removal rates are low; and ion exchange, using ion exchange resins to adsorb and remove impurities, followed by concentration and crystallization. This method has poor adaptability to raw materials, the resin is prone to clogging, and production efficiency is low. Summary of the Invention

[0004] This invention discloses a method for preparing electrowinning nickel using crude nickel sulfate, a byproduct of copper electrolysis, to solve any of the above-mentioned and other potential problems in the prior art.

[0005] To solve the above-mentioned technical problems, the technical solution provided by the present invention is as follows: a method for preparing electrolytic nickel using crude nickel sulfate, a byproduct of copper electrolysis, the method comprising the following steps:

[0006] S1) Neutral leaching: Crude nickel sulfate is immersed in tap water, stirred and filtered to obtain a crude nickel sulfate solution;

[0007] S2) Stir and heat the crude nickel sulfate solution obtained in S1) to a certain temperature, add a certain amount of oxidant such as sodium persulfate or hydrogen peroxide, adjust the pH value of the solution, react until no more precipitate is produced, filter, and obtain a preliminary purified solution.

[0008] S3) Add a certain amount of glutamic acid to the preliminary purified solution obtained in S2), stir at a constant speed and electrolyze to remove copper impurities, and obtain copper-removed solution.

[0009] S4) The copper-removed solution obtained in S3) is stirred at a constant speed and electrowinning is used to remove zinc impurities to obtain a purified solution.

[0010] S5) The purified liquid obtained from S5) is stirred at a constant speed and electrowinning is carried out to obtain high-grade electrowinning nickel.

[0011] Furthermore, the crude nickel sulfate in S1) is cobalt-containing nickel sulfate;

[0012] The neutral leaching process parameters are: temperature controlled at 70-90℃, and dissolution time at 25-35 minutes.

[0013] Furthermore, the molar ratio of the oxidant in S2) to the sum of cobalt and iron ions in the crude nickel sulfate solution is 2-4:1;

[0014] Adjust the pH of the solution to 5-5.5;

[0015] Stirring speed 450-550 r / min;

[0016] Heat to 70-90℃ and stir for 1.5-3.5 hours.

[0017] The purified liquid has a nickel ion concentration of 60–100 g / L and the concentrations of all impurity ions are below 0.005 g / L.

[0018] Furthermore, the oxidant is sodium persulfate or hydrogen peroxide.

[0019] Furthermore, the parameters of the electrowinning process in S3) are as follows: electrowinning temperature is 50-65℃, pH value is 3.5-4.5, and copper removal current density is 50-80 A / m. 2 The electrolyte needs to be stirred evenly at 100-200 r / min; the electrowinning time is 12 hours; and the concentration of glutamate in the solution should be controlled at 100-300 mg / L.

[0020] Furthermore, the parameters of the electrowinning process in S4) are as follows: electrowinning temperature is 50-65℃, pH value is 3.5-4.5, and current density is 180-230 A / m. 2 The electrolyte needs to be stirred evenly at 100-200 r / min; the electrowinning time is 36 hours.

[0021] Furthermore, the parameters of the electrowinning process in S5) are as follows: electrowinning temperature is 50-65℃, pH value is 3.5-4.5, and current density is 180-230 A / m. 2 The electrolyte needs to be stirred evenly at 100-200 r / min; the electrowinning time is 12 hours.

[0022] Furthermore, the anode in the electrowinning process is a Pb-Ag alloy plate covered with MnO2, and the cathode is a 316L stainless steel plate.

[0023] Furthermore, the method yields a nickel grade of not less than 99.92% and a total impurity content of less than 0.08%.

[0024] In an electroplated nickel, the electroplated nickel is prepared by the method described above.

[0025] Technical Principles

[0026] 1. Different metal ions exhibit different standard electrode potentials in solution: Cu 2+ / Cu+0.337、H + / H2±0.00、Pb 2+ / Pb-0.126、Co 2+ / Co-0.267、Ni 2+ / Ni-0.241、Fe 2+ / Fe-0.447、Zn 2+ / Zn-0.763.

[0027] 2.Co 2+ with Ni 2+ The standard electrode potentials are very close, making it unsuitable for purification by electrodeposition, while the K of Co(OH)2 is... sp 2.6*10 -16 K of Co(OH)3 sp 1.6*10 -44 Experiments comparing oxidation precipitation with sulfide precipitation or electrodeposition showed that nickel loss was the lowest.

[0028] 3. A similar principle applies to Fe. 2+ with Ni 2+ The standard electrode potentials are very close, and during electrodeposition, iron ions are oxidized to Fe at the anolyte. 3+ Cathodic reduction to Fe 2+ It consumes electrical energy, so it is not suitable for purification by electrodeposition. And Fe(OH)2 has K... sp 8.0*10 -16 K of Fe(OH)3 sp 4.0*10 -38Therefore, it can simultaneously oxidize iron and cobalt ions to remove two kinds of impurity metal ions; on the other hand, it can oxidize arsenic ions into insoluble arsenate for removal.

[0029] 4. The more positive the electrode potential of metal ions, the easier it is for them to be deposited or replaced and reduced at the cathode. In the nickel sulfate system, cobalt should not be replaced by zinc powder, otherwise it will be difficult to remove zinc ions in the later stage and will also increase the loss of nickel. Copper ions, which are more positive than nickel ions, can be purified by electrodeposition.

[0030] 5. Although zinc ions have a more negative standard electrode potential than cobalt ions, in fact, in the solution system prepared with nickel sulfate produced by copper electrolysis, zinc ions are more likely to be electrodeposited at the cathode than cobalt ions. This may be due to the decomposition of additives remaining in copper electrolysis into small molecule amino acids, which complex with zinc. In this process, the loss of nickel is less than that of sulfide precipitation.

[0031] 6. Lead anodes are prone to corrosion and contamination of the electrodeposition solution during electrodeposition. Therefore, it is necessary to cover the lead anode with a layer of MnO2 to protect it and to cover it with polyester cloth. The cathode should also be made of 316L stainless steel, which is more resistant to acids and alkalis.

[0032] 7. During electrodeposition, an oxygen evolution reaction occurs at the anode, and the pH of the electrodeposition solution decreases slowly over time. The hydrogen evolution reaction at the cathode gradually becomes dominant, reducing the cathode efficiency. Therefore, nickel carbonate is needed to adjust the pH of the system.

[0033] The method provided by this invention is simple and avoids the harm to the environment and people caused by using sodium sulfide or hydrogen sulfide under acidic conditions. By purifying and removing impurities from crude nickel sulfate containing multiple metals, it meets the requirements for nickel electrowinning, thereby obtaining high-grade nickel electrowinning. Attached Figure Description

[0034] Figure 1 The process flow diagram provided by this invention is for preparing electrowinning nickel from crude nickel sulfate, a byproduct of copper electrolysis. Detailed Implementation

[0035] The principles and features of the present invention are described below with reference to specific embodiments. The examples are only used to introduce a method for preparing electrolytic nickel using crude nickel sulfate, a byproduct of copper electrolysis, provided by the present invention, and are not intended to limit the scope of the present invention.

[0036] This invention provides a combined oxidation precipitation-electrodeposition method to prepare electrodeposited nickel with the aim of generating less waste gas, waste residue and waste liquid.

[0037] like Figure 1 As shown, the present invention provides a method for preparing electrolytic nickel using crude nickel sulfate, a byproduct of copper electrolysis, comprising the following steps:

[0038] S1) Neutral leaching: Crude nickel sulfate is immersed in tap water, stirred and filtered to obtain a crude nickel sulfate solution;

[0039] S2) Stir and heat the crude nickel sulfate solution to 80°C, add sodium persulfate or hydrogen peroxide as an oxidant, and adjust the pH of the solution to 5-5.5 with nickel carbonate. React until no more precipitates are produced, filter, and obtain a preliminary purified solution.

[0040] S3) Add glutamic acid to the preliminary purification solution, stir at a constant speed, and electrolyze to remove copper impurities.

[0041] S4) After copper removal, the solution is stirred at a constant speed and electrowinning is used to remove zinc impurities.

[0042] S5) The purified liquid is stirred at a constant speed and electrowinning is carried out to obtain high-grade electrowinning nickel.

[0043] The electrolyte obtained in S5) is returned to the system to leach crude nickel sulfate.

[0044] In this invention, the concentration of nickel ions in the purified liquid is 60-100 g / L, and the concentration of each impurity ion is less than 0.005 g / L.

[0045] In this invention, the electrowinning temperature is 50-65℃, the pH value is 3.5-4.5, and the copper removal current density is 50-80 A / m. 2 The current density for electrowinning zinc removal and nickel electrowinning is 180-230 A / m. 2 .

[0046] In this invention, the nickel concentration in the crude nickel sulfate solution is 80-110 g / L.

[0047] In this invention, the content of Fe ions, Co ions, Pb ions and As ions in the preliminary purified solution is less than 0.015 g / L.

[0048] In this invention, the impurity ions in the purified liquid are all below 0.005 g / L.

[0049] In this invention, the electrowinning process utilizes nickel carbonate to control the pH value of the system between 3.5 and 4.5.

[0050] In this invention, the nickel loss during the oxidation-hydrolysis precipitation process is <1%, the nickel loss during the electrowinning copper removal process is <2%, and the nickel loss during the electrowinning zinc removal process is <8%.

[0051] In this invention, the electrolytic lean solution is returned to the system to re-leach crude nickel sulfate.

[0052] In this invention, the anode in the electrodeposition is a Pb-Ag alloy plate covered with MnO2, and the cathode is a 316L stainless steel plate.

[0053] Example 1

[0054] Preparation of crude nickel sulfate solution: Dissolve 1.9 kg of crude nickel sulfate in tap water, stir at 500 rpm, maintain the temperature at 80℃, and dissolve for 30 minutes. Filter out the insoluble calcium sulfate to obtain 4.5 L of crude nickel sulfate solution. The composition of the crude nickel sulfate solution is: Ni 88.656 g / L, Cu 0.974 g / L, Zn 0.978 g / L, Co 0.338 g / L, Fe 0.452 g / L, Pb 0.022 g / L, As 0.088 g / L.

[0055] Oxidation water removes iron, cobalt, arsenic, and lead. A 4.5L crude nickel sulfate solution is heated to 80℃, and 20g of sodium persulfate is added. The mixture is stirred at 500 rpm for 2 hours, converting cobalt from divalent to trivalent. Arsenic precipitates as ferric arsenate and lead arsenate in the slag phase. The pH is adjusted to 5 with nickel carbonate, and the reaction continues for 30 minutes until no further precipitation occurs. Filtering yields 4.5L of preliminary purified solution. The preliminary purified solution composition is: Ni 88.456g / L, Cu 0.913g / L, Zn 0.956g / L, Co 0.011g / L, Fe 0.014g / L, Pb 0.005g / L, As 0.008g / L.

[0056] Copper removal by electrodeposition. 4.5L of preliminary purification solution was used as the electrolyte, with the electrolyte temperature controlled at 50℃; the electrodeposition current density was controlled at 50A / m. 2 The electrolyte needs to be stirred uniformly at 150 r / min; the electrowinning time is 12 hours; the glutamic acid concentration in the solution is controlled at 100 mg / L; the anode is a Pb-Ag alloy plate covered with MnO2, and the cathode is 316L stainless steel. To prevent corrosion of the anode lead plate and contamination of the cathode solution, the anode is treated with a polyester cloth bag diaphragm to obtain 4.5L of electrowinning copper removal solution. The composition of the electrowinning copper removal solution is: Ni 87.324 g / L, Cu 0.004 g / L, Zn 0.920 g / L, Co 0.006 g / L, Fe 0.008 g / L, Pb 0.003 g / L, As 0.002 g / L.

[0057] Electrodeposition to remove zinc. 4.5L of the copper-removed solution is used as the electrolyte, with the electrolyte temperature controlled at 50℃; the electrodeposition current density is controlled at 180A / m. 2The electrolyte needs to be stirred uniformly at 150 r / min; the electrowinning time is 36 hours; the anode is a Pb-Ag alloy plate covered with MnO2, and the cathode is 316L stainless steel. To prevent corrosion of the anode lead plate and contamination of the catholyte, the anode is treated with a polyester cloth bag diaphragm to obtain 4.5L of purified liquid. The composition of the purified liquid is: Ni 85.416 g / L, Cu <0.001 g / L, Zn 0.003 g / L, Co 0.003 g / L, Fe 0.002 g / L, Pb 0.003 g / L, As <0.001 g / L.

[0058] Nickel electrodeposition extraction. 4.5L of purified electrolyte was used as the following composition: Ni 85.416g / L, Cu <0.001g / L, Zn 0.003g / L, Co 0.003g / L, Fe 0.002g / L, Pb 0.003g / L, As <0.001g / L. The impurity composition met the requirements for nickel electrodeposition. The electrolyte temperature was controlled at 50℃, and the electrodeposition current density was controlled at 180A / m. 2 The electrolyte needs to be stirred evenly at 150 r / min; the anode is a Pb-Ag alloy plate covered with MnO2, and the cathode is 316L stainless steel. To prevent the lead plate of the anode from corroding and contaminating the cathode liquid, the anode is treated with a polyester cloth bag diaphragm.

[0059] The electrowinning nickel produced by the electrowinning process has a main grade of 99.92% and the sum of all impurities is less than 0.08%.

[0060] Example 2

[0061] Preparation of crude nickel sulfate solution: Dissolve 2.0 kg of crude nickel sulfate in tap water, stir at 500 r / min, control the temperature at 80℃, and dissolve for 30 minutes. Filter out the insoluble calcium sulfate to obtain 4.5 L of crude nickel sulfate solution. The composition of the crude nickel sulfate solution is: Ni 93.333 g / L, Cu 1.128 g / L, Zn 1.071 g / L, Co 0.397 g / L, Fe 0.497 g / L, Pb 0.093 g / L, As 0.115 g / L.

[0062] Oxidation of water removes iron, cobalt, arsenic, and lead. A 4.5L crude nickel sulfate solution is heated to 80℃, and 20g of sodium persulfate is added. The mixture is stirred at 500 rpm for 2 hours, converting cobalt from divalent to trivalent. Arsenic precipitates as ferric arsenate and lead arsenate in the solid phase. The pH is adjusted to 5 with nickel carbonate, and the reaction continues for 30 minutes until no more precipitates form. The solution is filtered to obtain 4.5L of preliminary purified solution. The composition of the preliminary purified solution is: Ni 93.217g / L, Cu 0.932g / L, Zn 0.916g / L, Co 0.013g / L, Fe 0.012g / L, Pb 0.009g / L, As 0.008g / L.

[0063] Copper removal by electrodeposition. 4.5L of preliminary purification solution was used as the electrolyte, with the electrolyte temperature controlled at 50℃; the electrodeposition current density was controlled at 50A / m. 2 The electrolyte needs to be stirred uniformly at 150 r / min; the electrowinning time is 12 hours; the glutamic acid concentration in the solution is controlled at 100 mg / L; the anode is a Pb-Ag alloy plate covered with MnO2, and the cathode is 316L stainless steel. To prevent corrosion of the anode lead plate and contamination of the cathode solution, the anode is treated with a polyester cloth bag diaphragm to obtain 4.5L of electrowinning copper removal solution. The composition of the electrowinning copper removal solution is: Ni 92.218 g / L, Cu 0.005 g / L, Zn 0.893 g / L, Co 0.007 g / L, Fe 0.006 g / L, Pb 0.002 g / L, As 0.002 g / L.

[0064] Electrodeposition to remove zinc. 4.5L of the copper-removed solution is used as the electrolyte, with the electrolyte temperature controlled at 50℃; the electrodeposition current density is controlled at 180A / m. 2 The electrolyte needs to be stirred uniformly at 150 r / min; the electrowinning time is 36 hours; the anode is a Pb-Ag alloy plate covered with MnO2, and the cathode is 316L stainless steel. To prevent corrosion of the anode lead plate and contamination of the catholyte, the anode is treated with a polyester cloth bag diaphragm to obtain 4.5L of purified liquid. The composition of the purified liquid is: Ni 90.149 g / L, Cu <0.001 g / L, Zn 0.003 g / L, Co 0.004 g / L, Fe 0.003 g / L, Pb 0.003 g / L, As <0.001 g / L.

[0065] Nickel was extracted by electrodeposition. 4.5 L of purified electrolyte was used as the following composition: Ni 90.149 g / L, Cu <0.001 g / L, Zn 0.003 g / L, Co 0.004 g / L, Fe 0.003 g / L, Pb 0.003 g / L, As <0.001 g / L. The electrolyte temperature was controlled at 50℃; the electrodeposition current density was controlled at 180 A / m. 2 The electrolyte needs to be stirred evenly at 150 r / min; the anode is a Pb-Ag alloy plate covered with MnO2, and the cathode is 316L stainless steel. To prevent the lead plate of the anode from corroding and contaminating the cathode liquid, the anode is treated with a polyester cloth bag diaphragm.

[0066] The electrowinning process produces electrowinning nickel with a main grade of 99.94% and a total impurity content of less than 0.06%.

[0067] The foregoing has provided a detailed description of a method for preparing electrolytic nickel using crude nickel sulfate, a byproduct of copper electrolysis, as provided in the embodiments of this application. The descriptions of the embodiments above are merely for the purpose of helping to understand the method and its core ideas; furthermore, those skilled in the art will recognize that, based on the ideas of this application, there will be changes in the specific implementation methods and application scope. Therefore, the content of this specification should not be construed as a limitation of this application.

[0068] Certain terms are used in the specification and claims to refer to specific components. Those skilled in the art will understand that hardware manufacturers may use different names to refer to the same component. This specification and claims do not distinguish components based on differences in name, but rather on differences in function. The terms "comprising" and "including" used throughout the specification and claims are open-ended and should be interpreted as "comprising / including but not limited to". "Approximately" means that within an acceptable margin of error, those skilled in the art can solve the technical problem and substantially achieve the technical effect within a certain margin of error. The following descriptions in the specification are preferred embodiments for carrying out this application; however, these descriptions are for the purpose of illustrating the general principles of this application and are not intended to limit the scope of this application. The scope of protection of this application shall be determined by the appended claims.

[0069] It should be understood that the term "and / or" used in this article is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. Additionally, the character " / " in this article generally indicates that the preceding and following related objects have an "or" relationship.

[0070] The foregoing description illustrates and describes several preferred embodiments of this application. However, as previously stated, it should be understood that this application is not limited to the forms disclosed herein and should not be construed as excluding other embodiments. It can be used in various other combinations, modifications, and environments, and can be altered within the scope of the application concept described herein through the foregoing teachings or techniques or knowledge in related fields. Any modifications and variations made by those skilled in the art that do not depart from the spirit and scope of this application should be within the protection scope of the appended claims.

Claims

1. A method for preparing electrowinning nickel using crude nickel sulfate, a byproduct of copper electrolysis, characterized in that, The method includes the following steps: S1) Neutral leaching: Take the raw material crude nickel sulfate, immerse it in tap water, stir and filter to obtain crude nickel sulfate solution; S2) Stir and heat the crude nickel sulfate solution obtained in S1) to a certain temperature, add a certain amount of oxidant, such as sodium persulfate or hydrogen peroxide, adjust the pH value of the solution, react until no more precipitate is produced, filter, and obtain a preliminary purified solution. S3) Add a certain amount of glutamic acid to the preliminary purified solution obtained in S2), control the concentration of glutamic acid in the solution to 100-300 mg / L, stir at a constant speed and electrolyze to remove copper impurities, and obtain copper-removed solution. S4) The copper-removed solution obtained in S3) is stirred at a constant speed and electrowinning is used to remove zinc impurities to obtain a purified solution; S5) The purified liquid obtained from S4) is stirred at a constant speed and electrowinning is performed to obtain high-grade electrowinning nickel.

2. The method according to claim 1, characterized in that, The crude nickel sulfate in S1) is cobalt-containing nickel sulfate; The process parameters for neutral leaching are: temperature controlled at 70-90℃, and dissolution time at 25-35 minutes; The nickel concentration in crude nickel sulfate solution is 80-110 g / L.

3. The method according to claim 1, characterized in that, The molar ratio of the oxidant in S2) to the sum of cobalt and iron ions in the crude nickel sulfate solution is 2-4:

1. Adjust the pH of the solution to 5-5.5; Stirring speed 450-550 r / min; Heat to 70-90℃ and stir for 1.5-3.5 hours; The purified liquid has a nickel ion concentration of 60-100 g / L and the concentrations of all impurity ions are below 0.005 g / L.

4. The method according to claim 1, characterized in that, The parameters for the electrowinning process in S3) are: electrowinning temperature of 50-65℃, pH value of 3.5-4.5, and copper removal current density of 50-80 A / m. 2 The electrolyte needs to be stirred at 100-200 r / min; the electrowinning time is 10-14 hours.

5. The method according to claim 1, characterized in that, The parameters for the electrowinning process in S4) are: electrowinning temperature 50-65℃, pH value 3.5-4.5, and current density 180-230 A / m. 2 The electrolyte needs to be stirred at 100-200 r / min; the electrowinning time is 32-38 hours.

6. The method according to claim 1, characterized in that, The parameters for the electrowinning process in S5) are: electrowinning temperature of 50-65℃, pH value of 3.5-4.5, and current density of 180-230 A / m. 2 The electrolyte needs to be stirred at 100-200 r / min; the electrowinning time is 10-14 hours.

7. The method according to any one of claims 4 or 5, characterized in that, The anode in the electrowinning process is a Pb-Ag alloy plate covered with MnO2, and the cathode is a 316L stainless steel plate. The actual substance used to regulate the pH value is nickel carbonate.

8. The method according to claim 1, characterized in that, The method yields electrolytic nickel with a main grade of not less than 99.92% and a total impurity content of less than 0.08%.

Images